The subject invention relates to a two-part frame and a pre-tensioning device that are useful for installing a taut sheet of material within a frame. Although any type of sheet material may be used within the frame, it is envisioned that the frame and pre-tensioning device will be most useful for preparing silk screens used by the printing industry.
To insure proper printing, a screen must be properly tensioned so that printing ink may pass therethrough in a crisp well-defined pattern. A screen which is too loose will cause an inconsistent pattern between printings and will result in poor definition of the printed design. On the other hand, a screen which is too tight will be subject to ripping, premature wear and a distortion of hole size, which causes uneven application of the ink upon the printed surface. Although many attempts at creating a properly tensioned screen have been employed in the past, no time efficient method has proved entirely satisfactory. Accordingly, one object of the subject invention is to create a properly tensioned frame in a fraction of the time required by prior art methods.
Herbert S. Levin, U.S. Pat. No. 2,903,967, issued Sep. 15, 1959, teaches a frame which stretches a wire mesh without causing punctures. In this invention, a rectangular frame is used. A vertical flange extends along the inner periphery of the rectangular frame and a screen is suspended between the edges of this vertical flange. The edges of the screen extending over the flange are clamped to a floating bar which extends substantially the length of each side of the frame exterior to the flange. The floating bar is connected to a screw and nut combination which allows the bar to be moved upward or downward perpendicular to the plane of the frame. By tightening a nut, the floating bar is raised upward thereby tightening the screen. By variably tightening a plurality of nuts, the screen may be tightened in a uniform fashion. Unfortunately, tension cannot be quantitatively determined and the tightening of the nuts, as well as the clamping of the screen, is done manually. Accordingly, the Levin frame is not suitable to large scale commercial operations.
Kaino Johannes Hamu, U.S. Pat. No. 3,962,805, issued Jun. 15, 1976, teaches a gripping device for a thin flexible piece of sheet material. A rectangular frame is used having a vertical flange which defines the interior perimeter of a rectangular frame. Channel members having a longitudinal channel of tampered cross-section are located exterior to the flange. The sheet material to be gripped is inserted through the channel and an anchor bar is placed therein to hold the sheet material within the channel. Gripper bars hold the channel members and are threadably mounted upon bolts that are secured through the frame. By turning the bolts, the gripper bars are pulled laterally away from or moved towards the center of the frame to adjust tension on the screen.
Todders, et al., U.S. Pat. No. 4,194,312, issued Mar. 25, 1980, teach a needlepoint supporting frame and clip assembly. Like other sheet material tensioning devices, the needlepoint supporting frame and clip assembly are rectangular in configuration. The sheet material in the Todders, et al. patent, is held in place by a plurality of clips. Tension in the frame is regulated by the movement of the four frame sides either toward or away from each other.
The subject invention provides a simple two-part frame which is useful for securing a piece of sheet material that is preferably presented in the pre-tensioned condition. To place a sheet material in this pre-tension condition, the subject invention provides a pre-tensioning device. The pre-tensioning device consists of a plurality of engaging means for gripping opposite ends of the sheet material to be secured within the frame. The engaging means are then retracted to place the sheet material in a taut condition. Typically, the tension upon the sheet material can be regulated by the amount of pressure used to retract the engaging means.
Through the use of an automated pre-tensioning device and two-part frame, a process which typically takes greater than two minutes, can be reduced to tens of seconds. This represents a major advance in the field of silkscreen technology which will ultimately result in a lower costs and improved quality.